The term membrane switches is generally used to refer to electrical switches constructed of two or more spaced layers of plastic film carrying conductive zones, areas or electrodes that are typically printed on the films with conductive inks. One film is arranged to be flexed or depressed toward another so as to establish an electrical circuit between specific electrodes, thereby generating a signal that can be detected by external circuitry to which the membrane switch is connected. The term membrane switches is also used in reference to switches having a single flexible plastic film of the foregoing type in combination with a rigid substrate or layer, each carrying conductive zones, areas or electrodes.
Membrane switches have become significant articles of commerce within approximately the last decade, and presently find widespread use in installations in which a sealed or protected switch or operating panel is a desirable component. Thus, membrane switches find use in equipment which requires manual data entry such as computer keyboards, terminals, cash registers and the like. Also, membrane switches are widely used as a control or instrument panel for appliances such as washers and microwave ovens, industrial controls, copy machines, and the like, in which a finger touch micromotion actuation is a useful feature.
Membrane switches are actuated simply by an operator depressing, with a finger, a selected or designated area of one layer of the switch towards another layer, the two layers being closely spaced from one another so that only a slight amount of movement of the layer which is depressed is required for proper operation.
Membrane switches have been primarily of the resistance type in which two contacts are closed and electric current flows through them in proportion to the voltage applied across them. Resistance switches are well known in the art and particular constructions for this type of membrane switch are disclosed in a number of patents assigned to the assignee of this application, including e.g., U.S. Pat. Nos. 4,217,473, 4,218,600 and 4,264,797.
A more recent development in the art of membrane switches is the capacitance type of membrane switch which operates upon the change of capacitance between spaced electrodes or contacts. In a capacitance membrane switch, a first set of capacitor plates or contacts is formed as a conductive pattern on the underside of a first film layer. A dielectric film layer is spaced from the first film layer and second and third capacitor plates are located on either side of the dielectric film layer. Assuming the second capacitor plates face the first capacitor plates on the underside of the first film layer, a variable air capacitor is established between the first and second capacitor plates and a fixed capacitor is established between the second and third capacitor plates, with the variable and fixed capacitors being in series with one another. When the conductive pattern on the underside of the first film layer and the third capacitor plates are connected to external circuitry, a detectable change in capacitance is developed when the first layer is depressed to establish contact between the conductive pattern on its underside and the second capacitor plates. Capacitance membrane switches of this general type are described in U.S. Pat. No. 4,367,385 assigned to the assignee of this application and U.S. Pat. No. 4,359,720 assigned to Honeywell Inc.
The research and development work that culminated in the present invention has for its objective the provision of a novel structure for capacitance membrane switches that would result in an improved switch construction and facilitate the manufacture of capacitance membrane switches. These and other advantages are explained in greater detail following the structural and electrical description of the present invention.